ITO (Indium Tin oxide) has been used for a transparent electrode in a transmission type liquid crystal device. However, since In2O3 of a main raw material of ITO contains a rare metal, and it has been feared whether a desired amount of ITO can also be steadily supplied in future. Japanese Patent Application Laid-Open No. 2000-044236 proposes a method of producing a new transparent conductor material which has a reduced content of In2O3, has a low electrical resistance, has an optical absorption edge in ultraviolet region and has an excellent blue light transmissivity; and an electrode material using the material.
In addition, materials which can be substituted for ITO have been actively researched and developed. For instance, Japanese Patent Application Laid-Open No. H07-235219 proposes a film of zinc oxide (ZnO) and a zinc-indium-based oxide. In addition, Japanese Patent Application Laid-Open No. 2000-044236 proposes an oxide containing zinc-indium-based oxide and a specified amount of an additive such as gallium.
In addition, there has been an attempt in recent years in which not only an electrode but also a channel layer (hereinafter, also referred to as “active layer”) of a transistor is formed of a transparent film. For instance, Japanese Patent Application Laid-Open No. 2002-76356 discloses a TFT (thin film transistor) which employs a polycrystalline thin film of a transparent conductive oxide containing ZnO as a main component, for a channel layer.
In addition, an organic electroluminescent light-emitting device has been actively researched and developed. When the organic electroluminescence is applied to a display unit, it is driven in an active matrix fashion in most cases. For the active matrix drive, a TFT which uses amorphous silicon or polysilicon is generally utilized.
In addition, Japanese Patent Application Laid-Open No. H09-114398 discloses an organic electroluminescent display unit which prevents degradation of a driving device, can display a television image, and aims at realizing a high-quality/high-luminance display unit. The organic electroluminescent display unit of an active matrix type according to this patent document is intended to prevent degradation of the driving device while keeping the high image quality, by using two MOS (metal oxide semiconductor) field effect transistors which use single-crystal silicon for an active layer.
In addition, Japanese Patent Application Laid-Open Nos. 2006-165528 and 2006-186319 disclose an example of producing a field effect transistor by using an amorphous oxide which is prepared by adding a specified amount of gallium or the like to a zinc-indium-based oxide to control the electron carrier concentration to less than 1018/cm3, and of applying the field effect transistor to an image display apparatus or a light-emitting apparatus.
In the conventional organic light-emitting/display apparatus, since amorphous silicon or polysilicon is used mainly as an active layer (channel layer) of a driving transistor, the gate insulating layer and the interlayer insulating layer can be used with no distinction. Further, since amorphous silicon or polysilicon used for the active layer originally contains hydrogen atoms, the diffusion of hydrogen in the insulating layers has posed no problem in most cases. Hydrogen in an a-SiN:H film produced by a plasma CVD (chemical vapor deposition) process has been rather considered to have an effect of compensating defects in amorphous silicon.
On the other hand, there has been found such a phenomenon that when an oxide semiconductor is used as an active layer of a driving transistor, there is a case where the transistor performs driving without any particular problem and a case where the off-state current increases during driving for a long period of time for unknown reasons. The phenomenon seems to depend on a method of forming an insulating layer which is in contact with an oxide semiconductor active layer, but the reason has not been clarified. Accordingly, there has been a problem that degradation of the characteristics may sometimes be caused due to the phenomenon. Moreover, the degradation of the characteristics has been intensively occurred during active matrix driving for a long period of time.
As described above, the conventional oxide semiconductor has the problem that when used in a stacked device, if a large amount of hydrogen is contained in an insulating layer, the degradation of the characteristics is liable to be occurred due to diffusion of the hydrogen. The problem has remarkably appeared in a display apparatus which employs the oxide semiconductor particularly for an active layer of a TFT. In this case, there has been a problem that a display apparatus cannot be realized which is stably driven for a long period of time and displays an image with high definition and less image defect.